A post-tensioning tendon is a complete assembly consisting of the anchorages, the pre-stressing strand or bar, the sheathing or duct and any grout or corrosion-inhibiting coating surrounding the pre-stressing steel. There are two main types of post-tensioning: bonded and unbonded. An unbonded tendon is one in which the prestressing steel is not actually bonded to the concrete that surrounds it, so its compressive force is transferred to the concrete by only its anchorages. The most common unbonded systems are monostrand tendons which are used in slabs and beams for buildings, parking structures and slabs-on-ground. A monostrand tendon consists of a seven-wire strand that is coated with a corrosion-inhibiting grease and encased in an extruded plastic protective sheathing. The anchorage consists of an iron casting in which the strand is gripped by a conical, two-piece wedge.
Typical strand Hydraulic Multi-strand Jack
Bonded systems are more commonly used in bridges, both in the roadway and in cable-stayed bridges. In buildings they are typically only used in heavily loaded beams such as transfer girders, where the large number of strands required makes them more economical.
In bonded strand systems, two or more strands are inserted into a metal or plastic duct that is embedded in the concrete. The strands are stressed with a large, multi-strand jack and anchored in a common anchorage device. The duct is then filled with a cementitious grout, which provides corrosion protection to the strand and bonds the tendon to the concrete surrounding the duct.